The kraft pulping of lignocellulosic materials leads inevitably to the formation of significant quantities of foul gases due to the reaction of sulfide-containing chemicals used in the kraft process with components in the lignocellulosic material. Collectively, these gases result in the foul smell associated with kraft pulp mills.
In addition to the unpleasant smell, the foul gases pose special problems because they are highly toxic, very corrosive, and explosive when mixed with air. These gases generally include hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide and are known as Total Reduced Sulfur (TRS) gases. Other noxious components of the foul gases include turpentine vapour and methanol. These foul gases are referred to collectively as noncondensable gases (NCG) since they cannot in general be condensed by common pulp mill vapour-cooling devices.
The chief sources of NCG emissions are well-known in the kraft pulping industry. Excluding the recovery boiler, a typical kraft mill has significant NCG discharges from many points including but not limited to the digester-blow and digester-relief areas, the multiple-effect evaporators, tall oil plant and filtrate tanks. The preferred treatment of these gases is by combustion of the gases in an incineration device. The lime kiln, typically used in the kraft process, is often preferred for combustion since it satisfies the thermal requirements for oxidation of sulfur gases and may allow sulfur recovery. However, other incineration devices are also suitable. These incineration devices include, but are not limited to, recovery boilers, power boilers and independent combustion furnaces as well as the lime kiln.
Other gases may be advantageously incinerated in a manner similar to the NCG stream. These include stripper gases derived from the air or steam stripping of evaporator condensates from the recovery system in the pulping process. Stripper vapours may include methanol, turpentines and other noncondensable gases.
In addition to the kraft process, the collection and incineration of noxious gases is also practiced in the Neutral Sulphite Semi-Chemical (NSSC) pulping process. In cases where both processes are carried out at the same mill site there is the potential for consolidating the NCG streams.
The major consideration in the design of systems to collect and burn the NCG is the explosive nature of the components. TRS gases are potentially explosive when mixed with air over the range of 2 to 50%. Turpentine is explosive at 1% and methanol at 7%. Furthermore, the flame propagation speed of TRS and methanol gases is about 0.5 m/s while that of turpentine is in the range of 150 m/s. Thus, special care must be taken in designing the system that collects, carries and incinerates these dangerous gases.
In conventional practice the NCG from the various sources are collected in a common pipe, and fans are used to blow the gas to the incineration device. This system has several problems, the most serious of which is the potential for fires and explosions should the fans produce an ignition source through sparks or hot spots. Other problems include corrosion of the fans by the gas, failure of fan seals, and resinous deposits on fans and other system components.
Recently, a system has been developed that uses steam ejectors as motivators to propel the gas directly to the incineration device obviating the problems associated with fans. Using steam for motivation eliminates the problems of sparks and hot spots, corrosion, and leaking seals. Furthermore, the steam can be introduced at a velocity above the flame propagation speed of the NCG stream regardless of the noncondensable gas flow rate.
Unfortunately, the use of steam ejectors has introduced a problem with regard to the incineration arrangement. The collected gas is normally directed through an injection nozzle into an incineration device such as the lime kiln. The extreme heat, often as high as 1200.degree. C. at the portion of the nozzle inside the kiln would result in rapid wear and failure of the nozzle were there not a system for cooling the nozzle during operation. In the fan-based NCG collection systems one known method to effect cooling is by using a water jacket surrounding the injection nozzle. However, attempts to adapt this efficient cooling system to the new safer systems designed with steam ejectors have been unsuccessful.
The application of the water jacket cooling system to the injection nozzle in the steam-based system results in the condensation of the steam that was used to motivate the NCG through the system. The water which collects in the cooled injection nozzle will drip into the kiln thus interfering with the kiln's smooth operation and even causing gaseous disruptions as the water droplets suddenly vaporize once again in the high heat of the kiln. The injection nozzle must therefore be cooled sufficiently to prevent nozzle failure yet its temperature must be elevated sufficiently to prevent the condensation of the steam entrained in the NCG stream.
One partial solution to the nozzle problem has been to insert a steam condenser in the steam-based NCG collection system in an attempt to recover heat from the steam/NCG stream. This condenser also removes a substantial portion of the motivating steam ahead of the cooled injection nozzle. In practice, this partial solution has been costly owing to the need for an additional condenser. Furthermore, efficient removal of steam also condenses some of the typically non-condensable gas components giving rise to a new effluent problem. Alternatively, partial prior condensation of the steam might leave the NCG stream intact, but the nozzle condensation problem would still be present. Most importantly, the gas velocity, after the steam has been removed, can fall to an unsafe level.
A second solution to the nozzle problem has been to install a combination of air and steam cooling in place of water cooling. This system suffers from disadvantages in that it is costly owing to the need for the additional fans and piping required as well as the need for sophisticated control and standby devices. Furthermore, the air and steam mixture is a relatively poor cooling media so that the nozzle must be constructed of expensive steel that can withstand the high temperatures found at the kiln exposed nozzle tip.
Thus, it can be seen that a serious problem exists in the art in the collection and combustion of noncondensable gases generated in a pulp mill.